feat(bench): support xvec and zvec backends

[zhangzhenghao]

Summary

• require an xvec or zvec backend argument and dispatch loading/searching to the selected implementation
• add a zvec-go backend for collection creation, batch insertion, optimization, and concurrent/serial HNSW queries
• include the backend and backend-specific label in JSON reports
• update the benchmark guide with the requested xvec command and an equivalent zvec-go command
• keep normal CGO builds lightweight through an explicit stub; build with CGO_ENABLED=0 to produce one binary supporting both backends

Verification

• go test ./... -count=1
• go vet ./...
• CGO_ENABLED=0 go vet ./cmd/vector-db-bench
• CGO_ENABLED=0 go test ./cmd/vector-db-bench -count=1 with ZVEC_LIBRARY_PATH pointing to the official zvec-go v0.6.0 Linux x86-64 C API library
• real custom Parquet end-to-end load, optimize, concurrent search, serial search, and Recall@1 checks passed for both xvec and zvec
• requested Cohere 1M xvec and zvec command lines validated with --dry-run

The full 1M dataset benchmark was not run locally because it downloads roughly 3.1 GB of training data and is the workload this command is intended to launch.

[codecov]

Codecov Report

❌ Patch coverage is 92.72727% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.35%. Comparing base (c8e0544) to head (d11c826).

Files with missing lines	Patch %	Lines
cmd/vector-db-bench/benchmark.go	78.57%	2 Missing and 1 partial ⚠️
cmd/vector-db-bench/config.go	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
- Coverage   61.35%   61.35%   -0.01%     
==========================================
  Files         147      149       +2     
  Lines       40203    40240      +37     
==========================================
+ Hits        24668    24689      +21     
- Misses      12322    12340      +18     
+ Partials     3213     3211       -2     

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